When intense electric field is applied to the surface of metal, potential level is inclined at the vacuum. Then when the electric field exceeds the order of 109 V/m, the potential barrier becomes extremely thin, so that electrons are emitted into vacuum due to the tunneling-effect. This is called field emission. When intense electric field with the order of over 108 V/m is applied to the surface of heated metal, the Schottky effect occurs, and then electrons are emitted into vacuum. Recently as the electron source of a high-resolution electron microscope, a field-emission electron source based on the field emission phenomenon (Cold Field Emitter: CFE) and a Schottky electron source based on the Schottky effect (Schottky Emitter: SE) are mainly used.
In order to emit an electron beam from an electron source, such as CFE and SE, the diameter of the electron source at the tip end has to be thin in the nanometer order because sufficiently intense electric field is applied to the tip end. Conventionally a typical electron source is manufactured by sharpening a metal thin wire at the end by electrolytic polishing. Electrolytic polishing is a technique to sharpen the metal wire in electrolyte, applying voltage for electrolysis to melt the metal wire. In case of using SE, it is required to have a proper diameter of electron source at the tip end according to the property such as the amount of the beam current, the width of the beam energy, the stability of the beam and so on. And as other methods to sharpen the wire, there are various methods such as heat-treating, dry etching and ion etching in order to have the desired diameter.
JP H11-31453 A, for example, discloses a method of manufacturing a tip by electrolytic polishing. JP H08-36981 A discloses a method of heat-treating of a sharpened thin wire, dry etching, and etching by the ions which are ionized gas by electrons, which are emitted from tungsten monocrystal wire by applying voltage as a cathode. JP 2008-177017 A discloses a method of processing a tip end by FIB.